1. Field of the Invention
The present invention relates generally to a transfer of thermal energy, and more particularly, to heat transfer arrangements employing a membrane desiccation heat pump for both heating and cooling applications.
2. Description of the Prior Art
A cooling of a gaseous fluid containing humidity and, in particular, a cooling of humid air, is desired in many circumstances. Cooling of ambient atmosphere is often desired in buildings, domestic dwellings, in appliances such as refrigerators, and in storage rooms and the like. It is also desired in delivery vehicles and trucks, and in aircraft and marine craft. Other applications are natural gas cooling for removal of natural gas liquids or providing of controlled inert atmosphere to industrial processes such as paint drying, food drying and clean rooms.
Cooling systems using water vapor as working fluid are among the oldest in the art of producing cold. Early processes made use of principles of both compression and adsorption refrigeration, the latter using sulfuric acid as an absorbent. Vacuum refrigeration systems using water vapor or organic vapor as a refrigerant and steam injector as a compressor were well adapted to air conditioning application in the 1940s.
Many different proposals have been made for cooling gases and, in particular, for the cooling of air. A popular system in widespread use utilizes a compression and expansion of a heat exchange medium in the form of a gas, which can be compressed into a liquid state, and then is allowed to expand into a vapor state, i.e., the so-called compression/expansion cycle. In most cases, chloro-fluoro carbon gases, e.g., Freon(trademark), were used, but recently such gases have been considered environmentally unsafe. Conventional alternatives to Freon(trademark) are not as efficient as Freon(trademark), and thus systems using the compression-expansion cycle require a relatively large input of power for the compression cycle. As a result, attention has turned to the feasibility of air conditioners that rely on alternative energy sources. Desiccant air conditioning systems are able to utilize alternative sources such as waste heat or solar energy for cooling and air conditioning thereby reducing electric power consumption and reducing reliance upon conventional power sources.
Desiccant air conditioners work as follows. During a cooling mode or adsorption cycle, hot humid air enters an intake side of an air conditioning system and passes through one side of a slowly turning desiccant wheel or circular desiccant bed. Water vapor and other moisture vapor are adsorbed on an extended desiccant material surface area, drying the air and releasing latent heat of condensation. Hot dry air from the desiccant bed wheel then passes through a heat exchanger such as an air-to-air heat exchanger wheel giving up some of the heat to an exhaust air stream. The air is then reconditioned to be in a desired comfort zone by passing through an evaporative element or unit where moisture is evaporated back into the air, for example by spraying, cooling the air to a desired temperature and humidifying the air to a desired relative humidity.
Open cycle desiccant systems have been known from the early 1940xe2x80x2s. In 1955, U.S. Pat. No. 2,700,537 to Pennington described using rotary heat exchangers impregnated with desiccants. Today dual path machines still use the Pennington cycle. In 1960, U.S. Pat. No. 2,926,502 to Munters improved this cycle. The ""502 patent discloses an air conditioning system including the recycling of air, at least three air flow paths, with all embodiments including a recycling of interior space conditioned air path, an open cycle regeneration path and a supplementary air path for an additional heat exchanger.
U.S. Pat. No. 4,594,860 to Coellner et al. discloses an open cycle desiccant air conditioning system in which the regeneration path is an open cycle and very similar to Pennington""s cycle. U.S. Pat. No. 2,186,844 to Smith discloses a refrigeration apparatus wherein heat from a mechanical refrigeration unit regenerates desiccant, very similar to concepts described in U.S. Pat. No. 5,502,975 to Brickley et al. and U.S. Pat. No. 5,517,828 to Calton et al. The common factor is the open cycle regeneration path. U.S. Pat. No. 4,786,301 describes an air conditioning system having heat exchanging desiccant bed with alternating adsorption desorption cycles, an improvement of this concept is described in U.S. Pat. No. 5,222,375 entailing the use of two alternating desiccant beds.
U.S. Pat. No. 5,353,606 to Yoho et al. addresses a three-path desiccant air conditioning system. As with other prior art systems, all these regeneration paths are open cycle.
Some of the problems associated with desiccant air conditioners are the need for removal of latent heat of condensation and adsorption from the desiccant bed and desiccant material during the adsorption cycle, the need for thermal energy for the desiccant regeneration cycle and the need to cool the desiccant after the regeneration. Furthermore, desiccant wheel machines are cumbersome to build and require a high level of maintenance.
Several authors have suggested employing a heat pump to raise the temperature of heated fluid for utilization of a waste heat stream. Heat pump systems employed for such processes are based on conventional cycles, i.e., fluid compression, absorption, sorption and desiccation cycles. The use of heat pumps for heat recovery have been shown in several applications utilizing various low level heat sources such as heat emitted by refrigerators (U.S. Pat. Nos. 4,041,724 and 4,226,089), an exhausted air duct (U.S. Pat. Nos. 4,100,763, 4,175,403 and 4,416,121), paper mill processes (U.S. Pat. Nos. 4,026,035, 4,437,316, 4,522,035 and 4,780,967), a power plant (U.S. Pat. No. 4,124,177), solar energy (U.S. Pat. Nos. 4,143,815, 4,332,139 and 4,703,629). Other sources of low level heat are: a plurality of secondary heat sources from an industrial plant or a factory (U.S. Pat. Nos. 4,173,125, 4,307,577, 4,333,515 and 5,548,958.); humid air (U.S. Pat. Nos. 4,197,713 and 4,517,810); air exhausted from a paint spray booth (U.S. Pat. No. 4,197,714); a gas stream of a drying oven (U.S. Pat. No. 4,295,282); a building stack or a flue (U.S. Pat. Nos. 4,314,601 and 4,660,511); waste heat from a gas turbine (U.S. Pat. No. 4,347,711); thermally activated separation processes such as fractional distillation, distillation, dehydration, or acid gas scrubbing (U.S. Pat. Nos. 4,347,711 and 5,600,968); waste water heat (U.S. Pat. No. 4,448,347); fumes from a heating boiler (U.S. Pat. No. 4,523,438); boiling solvent vapor (U.S. Pat. Nos. 4,537,660 and 4,539,816); waste heat heated water (U.S. Pat. No. 4,819,446); waste heat such as absorber heat, hot vapor heat, flue gases, or a combination thereof (U.S. Pat. No. 5,255,528).
A membrane separation method for removing water vapor from a gas is a method wherein a gas containing water vapor is contacted to one side of a vapor permselective membrane assembly, and a dry gas is contacted to the other side of the membrane, so that the water vapor is selectively permeated and separated through the membrane. In principle, it has merits over other three methods such that the running cost is low, the structure of the apparatus is simple, and dry air can continuously be obtained without polluting air. As a vapor permselective membrane excellent in permeability of water vapor, an ion exchange membrane as well as a dehumidifying method using such a membrane has been proposed by U.S. Pat. Nos. 3,735,558 and 4,909,810. Hollow fiber membrane-based dehydration is also known. See, for example, U.S. Pat. Nos. 4,783,201, 4,725,359, 4,718,921, 4,497,640, 4,583,996 and 3,511,031. U.S. Pat. No. 4,900,626 discloses a hollow composite fiber for dehydration having a polydimethylsiloxane coating on a dense layer of the fiber support.
Although membranes have been used in various separation applications, their use for heat pump systems has been limited. U.S. Pat. Nos. 4,152,901 and 5,873,260 propose to improve an absorption heat pump by using a semi-permeable membrane and a pervaporation membrane, respectively. U.S. Pat. No. 4,467,621 proposes to improve vacuum refrigeration by using a sintered metal porous membrane and U.S. Pat. No. 5,946,931 shows a cooling evaporative apparatus using a microporous PTFE membrane.
It is an object of the present invention to provide a membrane desiccation heat pump system that is useful for various application such as cooling, chilling, heating and air conditioning, and that is low-cost, efficient and simple to maintain.
Another object of the present invention is to provide such a membrane desiccation heat pump system, characterized by an energy economy or efficiency better than that of prior art systems. One important feature of the present invention is that it employs a process carried out without an addition of costly heat energy from outside for separating a sorption agent from a working medium.
Another object of the present invention is to provide a membrane desiccation air conditioning system, which permits a greater degree of independent control of temperature and humidity for achieving parameter levels in desired regions of the comfort zone than prior art systems.
A further object of the present invention is to provide an open system membrane desiccation air conditioner having a broad range of applications and being capable of net introduction of either heat or mechanical energy from an external source and net removal of heat energy relative to the air conditioning system for greater flexibility and control of comfort zone parameters.
Additional objects of the present invention are to provide a membrane desiccation based liquid chilling and heating process systems usable for various applications.
Yet another object of the present invention is to provide a new method of waste or low level energy recovery using a membrane desiccation-based heat pump process adapted to extract unusable thermal energy from a media at low temperature and upgrade it to a higher, useful temperature.
A first embodiment of the present invention is a system for pumping thermal energy. The system includes (a) a membrane permeator for removing vapor from a process gas and for providing a vapor-depleted process gas, and (b) a gas-liquid contactor for adding vapor from a liquid to a the vapor-depleted gas to produce a vapor-added process gas. The system transfers a quantity of thermal energy from the liquid to the vapor-added process gas.
A second embodiment of the present invention is a system for pumping thermal energy including (a) an energy source for heating a vapor-containing gas, and (b) a membrane permeator for removing vapor from the vapor-containing gas and for providing a resultant vapor. The system transfers a quantity of thermal energy from the energy source to the resultant vapor.
A third embodiment of the present invention is a system for pumping thermal energy including (a) a heater for heating a liquid, (b) a gas-liquid contactor for adding vapor from the liquid to a process gas to produce a vapor-containing gas, and (c) a membrane permeator for removing the vapor from the vapor-containing gas and for providing a resultant vapor. The system transfers a quantity of thermal energy from the heater to the resultant vapor.
The present invention also contemplates methods for employing the systems described herein.
A first method for pumping a quantity of thermal energy includes the steps of (a) adding vapor from a liquid to a process gas, thus yielding a vapor-containing gas, and (b) employing a membrane permeator to remove the vapor from the vapor-containing gas, thus providing a resultant vapor. The quantity of thermal energy is transferred from the liquid to the resultant vapor.
A second method for pumping a quantity of thermal energy includes the steps of (a) employing a heater to heat a vapor-containing gas, and (b) employing a membrane permeator to remove vapor from the vapor-containing gas, thus providing a resultant vapor. The quantity of thermal energy is transferred from the heater to the resultant vapor.
A third method for pumping a quantity of thermal energy includes the steps of (a) employing a heater to heat a liquid, (b) adding vapor from the liquid to a process gas to produce a vapor-containing gas, and (c) employing a membrane permeator to remove the vapor from the vapor-containing gas, thus providing a resultant vapor. The quantity of thermal energy is transferred from the heater to the resultant vapor.
A fourth method for pumping thermal energy includes the steps of (a) removing vapor from a process gas with a membrane permeator to yield a vapor-depleted gas, and adding vapor from a liquid to a the vapor-depleted gas via a gas-liquid contactor to yield a vapor-added gas. The method transfers a quantity of thermal energy from the liquid to the vapor-added gas.
The present invention involves a membrane desiccation heat pump for use in association with a gaseous flow system adapted to move a gaseous process fluid, such as air, containing a quantity of moisture vapor content. In a preferred embodiment, the cooling system includes (a) humidification means, e.g., a gas-liquid contactor, operable to add vapor of a vaporizable liquid fluid to a process gaseous fluid, (b) a moisture vapor membrane permeator, (c) means for passing the process fluid through the membrane permeator to dry the process fluid and substantially reduce the quantity of moisture vapor in the process fluid, thereby separating the vapor from the gaseous process fluid and (d) vapor removal means for removing the concentrated vapor from the from the membrane permeator. An evaporation process utilizes heat from one of the fluids or from both for the latent heat of vaporization of the liquid, to reduce the temperature thereof and simultaneously establish a desirable humidity level in such gaseous process fluid.
One feature and advantage of the present invention is that the membrane desiccation heat pump permits a substantial import of energy from an external source, such as an alternative or low level energy source, into the system, and a substantial removal of heat energy from the system. Thus, the present invention provides a very efficient heat pump both for the purpose of either cooling or heating.
The liquid-gas contactor facilitates mass transfer of liquid vapor from a liquid into a gas while simultaneously removing latent heat from the liquid and thereby reducing the temperature of both. It also acts as a direct-contact heat exchanger that enables a heat transfer from a fluid that has to be cooled to a circulating fluid. It may be any unit that operates on a principle common in the industry such as spray or atomizing, dripping, sprinkler, wet pad, packed column, plates column, baffle tower or any type of cooling tower. It could also be a membrane contactor having any of the configurations described above for the membrane permeator assembly, however, in this case, it would enable the evaporation from the liquid to the gas phase. Other possible embodiments for such a contactor include a humidifier or any type of evaporator, for example, a short tube evaporator or a long tube evaporator, or a flash evaporator.
A membrane separator removes vapor from a stream of carrier gas. The removal of the vapor from the gas stream is also involves a removal of the latent heat contained in the vapor, thus providing a thermodynamic vehicle for a heat pump.
For use in the present invention, a membrane separator assembly is preferably capable of an efficient and economical separation of vapor from the carrier gas. Such a membrane separator assembly may come in different shapes and forms. The following paragraphs describe several features of the membrane assembly that may be used for vapor separation in the present invention:
The membrane separator assembly may include one or more membrane units that could be assembled in any array, such as series, parallel, cascade or in a membrane column. A membrane unit may be either a self-contained module or a combination of one or more membrane elements in a housing. A membrane element can be any form of packaging of membrane area in a single item such as but not limited to a module, a cartridge, a plate, etc.
The membrane can be solid or liquid, organic or inorganic, pre-made or dynamic, charged or uncharged, ionic or non-ionic, hydrophilic or hydrophobic, porous or dense, permeable, non-permeable or semi permeable. It can be polymeric, metallic, ceramic, carbon or glass. Membrane geometry can be in any form such as a flat sheet, tubular, capillaries, hollow fibers or a monolith. A membrane element can be in any configuration such as hollow fiber module, hollow fiber plates, spiral wound module, plate and frame, pleated or folded cartridge, envelopes, bags, tubes and sheets, spiral tubes, candles or monolithic.
Flow patterns of either a feed-retentate side or a permeate side of the membrane in the membrane unit may be countercurrent, co-current or both. It could be transverse flow, diagonal flow or random direction flow. It could be unidirectional or multi directional. It could be one pass or multi pass. Either flow could be on any side of the membrane in the element configurations detailed above.
Removal of the permeating vapor from the membrane unit may be direct or it could be facilitated by means such as reflux flow, either internal or external, vacuum and/or condensation. It could be a sweep stream that may be of gas or liquid. Such a sweep stream may be inert or reactive, i.e., having either physical or chemical affinity toward the permeating vapor.
A good membrane process for vapor separation from gas especially dehydration must be capable of removing water vapor from the feed gas to the desired dew point. Preferably, the water vapor separation takes place with as little loss of feed gas to the permeate as possible and the process must economically perform this separation; in other words, the membrane surface area required to perform a given water vapor separation should be as small as possible.
Vapor permeates through a membrane from a feed side of the membrane to a permeate side of the membrane. A driving force for a transport of vapor through the membrane is a partial pressure differential of the vapor across the membrane. Therefore, a partial pressure of water vapor in a gas leaving as permeate from a membrane module cannot exceed a partial pressure of water vapor in a feed gas entering the membrane module. In most cases, the partial pressure of water in a gas at saturation is very low. Therefore, the partial pressure driving force for the vapor transport must be provided by one of three methods: (1) a sweep method, in which dry gas from an external source is swept proximate to the permeate side of the membrane; (2) a vacuum method, in which a vacuum is applied to the permeate side of the membrane; or (3) a dilution method, in which (a) the permeate is left at, for example, atmospheric pressure, but either a small percentage of the feed gas is allowed to permeate the membrane, diluting the water vapor content of the permeate, or (b) reflux of a part of the dry retentate into the permeate compartment. In all three cases, the driving force for the permeation of the vapor in the feed gas is provided by the vapor partial pressure difference between the feed and the permeate.
The membrane desiccation heat pump can be constructed in several configurations. It may be either an open cycle or closed cycle gas system, and it may be either an open cycle or closed cycle liquid system. The system may be operated either in a cooling mode or in a heating mode. It may also determine both the temperature and the vapor content of the out put gas. Specifically, the membrane desiccation heat pump may serve as a very efficient air condition system both for large and small sizes, and it may be an excellent system for providing chilling water for various applications.
As in any heat pump, the laws of thermodynamics govern the membrane desiccation heat pump. It removes heat from a media at a low temperature and transfers it to a higher temperature by investing external energy into the process. Commonly, a compression heat pump requires mechanical energy input, and an absorption heat pump requires thermal energy input to facilitate the thermodynamic process. A membrane desiccation heat pump can utilize either form of energy input or both together.